The present invention relates in general to police radar detectors and, more particularly, to input stages for police radar detectors which provide improved noise figure and sensitivity for police radar detectors.
Police have used radar waves to monitor the speed of motor vehicles for many years. The frequencies currently licensed in the United States for police radar waves include: the X band--10.50 gigahertz (Ghz) to 10.55 Ghz; the K band--24.05 Ghz to 24.25 Ghz; and, the K.sub.a band--33.40 Ghz to 36.00 Ghz. To alert motorists of the presence of police radar, electromagnetic signals within these police radar bands are monitored using a police radar detector which issues an audible and/or visual alert upon detection of a radar signal within one of the bands.
One way to improve the effectiveness of a police radar detector is to provide the highest sensitivity possible so that the radar detector will provide the earliest possible warning to an operator of the detector. The front end circuitry or input stage of most police radar detectors is "passive". An antenna receives incoming electromagnetic signals and delivers them to a first mixer of a superheterodyne receiver. The first mixer is normally a diode or diodes pumped by a first local oscillator with the first mixer output being delivered to a first intermediate frequency (IF) amplifier. The noise figure of the receiver on any given frequency is the noise figure of the first mixer at that frequency plus the noise figure of the remainder of the system measured at the first IF input. Noise figure improvement of the system can be attempted by several known methods.
For example, the noise figure at the first IF amplifier input can be minimized. Well designed receivers have IF noise figures reduced to reasonably low levels so that little further improvement is practical. The mixer noise figure is generally the dominant contributor to the overall police radar detector system noise figure and minimizing mixer conversion loss receives manor attention in receiver design. A well-known palliative for coping with mixer loss is to precede the first mixer with a radio frequency (RF) preamplifier. However, the successful application of this principle to police radar detection has been elusive.
A readily apparent application of an RF preamplifier to a police radar detector is to place a broadband preamplifier with good noise figure and sufficient gain between the antenna and the first mixer of a radar detector system to seemingly render the conversion loss of the first mixer insignificant. Unfortunately, such a simplistic application of a preamplifier results in impaired performance. The problem is that broadband preamplifiers produce broadband noise and typical radar detectors have multiple receiver frequencies or responses so that off channel noise is received. Because of the lack of selectivity, off channel preamplifier noise contributes to the receiver noise floor, resulting in loss of sensitivity to the desired signal.
Modern radar detectors must scan wide bandwidths to cover the police radar bands licensed in the United States which are noted above. Accordingly, an important aspect of police radar detectors is the selection of the frequencies used within the detectors, i.e., frequencies or swept frequency bands of local oscillators and frequencies of intermediate amplifiers and filters, referred to in the art as frequency schemes. Many different frequency schemes are used by detector manufacturers, but an known detectors make use of multiple frequency conversion responses to simpify coverage of the wide bandwidths that must be scanned. Many frequency schemes use a first IF near 1 Ghz.
It is apparent that the typical police radar detector frequency scheme has numerous instantaneous receive frequencies, often four or even six. Presume that the mixer and preamplifier performance are the same at each frequency, which is desired for receiving radar signals. The dilemma of using an RF preamplifier is that the desired signal arrives via only one of the possible instantaneous receive frequencies but preamplifier output noise arrives via all possible receive frequencies, four or six responses. The noise power is thus four or six times the power that would be received on a single response receiver. Stated differently, the noise temperature of the receiver is four to six times (6 dB to 8 dB) higher than would be achievable in a single response system. The same lack of front end selectivity that is so beneficial in providing simple, economical frequency coverage compromises system noise temperature when a broadband preamplifier is used.
What is required to achieve good noise performance is RF selectivity (often called preselection, accomplished with a preselector) between the preamplifier and the mixer so that only the response delivering the signal contributes noise from the preamplifier. Preselection is generally used to achieve "image suppression" and, in radio receivers which are more typical than police radar detectors, serves to suppress reception of undesired RF signals as well as enhance noise figure performance. However, in police radar detector designs, many if not all the "image" responses are useful for reception of radar signals and the only incentive for applying RF selectivity is to attain improved noise figure when RF preamplifiers are being utilized. Optimum application of a RF preamplifier to radar detectors thus requires careful management of the noise paths that arise in the radar detector's frequency scheme.
In the case of radar detectors, the preselector must be dynamically tuned to the desired receive frequency as the detector scans the various radar bands. Tunable preselectors have traditionally been implemented by using PIN diodes to select among a variety of filters or by using PIN diodes or varactors to tune reactances within a single filter network. These approaches can be cumbersome and expensive to implement over extended bandwidths at microwave frequencies.
RF preamplifiers have not been generally adopted in police radar detectors. The only known police radar detector using RF preamplification is commercially available from the assignee of the present application and is taught in the assignee's U.S. Pat. No. 5,068,663. The '663 patent discloses a radar detector using a preamplifier on the X band to achieve improved sensitivity on that band with the K and K.sub.a bands being handled passively. In the '663 implementation, one mixer/IF path processes the X-band and a second mixer/IF path processes the K/K.sub.a band signal path so that two mixers are required for the input stage of the police radar detector.
What is needed is a simplified and economical input stage for police radar detectors which satisfies the requirements for frequency preselection to improve the noise figure and sensitivity of police radar detectors. Preferably, the improved input stage would utilize a single mixer.